Many devices exist for performing an operation on a moving web wherein a gas or gas pressure differential is applied. For example, in the art of papermaking, a wet or embryonic paper web can be partially dried or dewatered by means of an applied gas pressure differential using nozzles, a compressive roll with holes or grooves for pressurized air, an xe2x80x9cair pressxe2x80x9d or other devices wherein a gas pressure differential forces air into a web to displace liquid water and/or to remove water by evaporation. Many prior systems, particularly those having compressive rolls, impose high compressive force on the web and are not suitable for many low density materials such as tissue. Systems of nozzles are typically inadequate due to the low residence time provided for air penetration into the web. Nozzle systems also either suffer from high leakage if the nozzle is not in contact with the web or from high fabric wear if the fabric wraps the surfaces of the nozzle to provide some degree of sealing. A fabric wrapping a nozzle with a small effective radius of curvature is particularly likely to experience wear problems.
Flat pressurized boxes, such as steam boxes, while capable of good residence time in some cases, suffer from high leakage from the sides of the steam chamber. Steam boxes for heating paper webs in particular have tended to be flat boxes with a finite gap between the web and the sides of the box. Such gaps or clearances allow significant volumes of air to enter, in part due to the air boundary layer traveling with the web. Intentionally bleeding steam to oppose the boundary layer or using a steam curtain to prevent entry of the boundary layer is inherently inefficient.
Recognizing the difficulty of providing adequate seals in pressurized paper drying chambers, some have proposed the use of high-velocity heated air impingement that relies on the momentum of the air to push through the web for paper drying without attempting to use seals. This technique is intended to minimize lateral migration of the drying air along the surface of the web, thereby reducing the need for sealing. Even if this method reduces the lateral flow of air, the extent of treatment is limited by the brief contact time of the pressurized gas with the web due to the narrow jets employed. Without suitable seal elements, leakage still will not be prevented.
Rotary devices, such as cylindrical through dryers and suction rolls, can be operated to pass air through a fibrous sheet but are complex and costly devices. Further, the surface of the rotary device or other supporting surfaces in contact with the web have significant closed areas where gas flow is blocked, resulting in nonuniform penetration of the gas through the web.
An excellent system for effective gas treatment of a moving web is the dewatering system disclosed in commonly owned copending application Ser. No. 08/961,915 of Hada et al. filed Oct. 31, 1997 and incorporated herein by reference in its entirety. This application teaches an air press wherein a non-rotating upper plenum is used to apply pressurized air to a moist paper web while the web is sandwiched between two pervious fabrics. The pressurized plenum cooperates with a lower vacuum box on the opposite side of the sandwiched paper web such that the overall pressure differential across the web is greater than if the pressurized plenum were used alone at a predetermined pressure. An important issue in the operation of an air press is preventing unnecessary leakage of gas out of the plenum into the surrounding atmosphere. Hada et al. disclose a set of cross-direction seals (seals running in the cross-direction) for the leading and trailing edges of the plenum (the leading edge being closest to the headbox) and a pair of machine-direction seals running in the machine direction to seal the side edges of the plenum. Hada et al. also disclose a lever system for increasing sealing force on the seals responsive to measurement of air pressure in the plenum. The principle of operation is that excessive leakage will result in a reduction of pressure in the plenum, which can then be compensated by increasing the applied pressure to the systems of seals.
Though capable of opposing leakage and preventing large leaks, such a system may lead to excess fabric wear, particularly in wide machines, because sealing is generally performed across the entire width of the machine, rather than solely in the locations where leak reduction is needed. Further, use of internal pressure measurements to detect leak control may lead to some false readings of leakage when pressure fluctuates for other reasons, such as changes in web properties or compressor operation or air temperature. Further still, a single macroscopic measure of pressure cannot be used to locate specific leaks, only leakage in general. Therefore, what is lacking and needed is a method and apparatus for detecting the specific location of leaks and applying corrective remedies to prevent leakage only at the locations where leakage is occurring.
Pressurized or depressurized web treatment chambers for processing moving webs can operate more efficiently if fluid leaks are detected and controlled with localized leak detection means and with localized leak reduction means, wherein the leak reduction means is operatively responsive to the leak detection means such that local leaks are effectively sealed or reduced in severity.
In particular, a control method has been discovered for web treatment systems for moving webs, the treatment system involving at least one chamber at a pressure substantially different from the ambient pressure, wherein localized leaks are detected by one or more leak detectors and wherein the leaks are reduced by application of pressure reduction means responsive to the one or more leak detectors. The control method of the present invention can provide improved means for prevention of leaks in web treatment systems of all kinds wherein a moving web passes through a pressurized treatment chamber.
Further, the control system of the present invention can be used to prevent leakage of chemicals from a web treatment chamber or to prevent excess infusion of atmospheric air or oxygen into web treatment chamber by means of localized leak or infusion detectors operatively associated with localized leak reduction means to apply improved sealing in the localized regions where such action is needed. Such a system is desirable when the fluid in the web treatment chamber has a substantially different chemical composition than the ambient atmosphere and it is desired to prevent leakage of the treatment fluid into the atmosphere or to prevent leakage of air into the chamber. Again, it is desirable to maintain effective seals across substantial distances as a web passes through the seals through a combination of localized leak detection and reduction, rather than subjecting the entire apparatus or entire extent of the web and the seals to increased clamping pressures. As used herein, the term xe2x80x9cleakxe2x80x9d encompasses both the escape of fluid from within the chamber (e.g., the escape of pressurized air from an air press) and the infusion of fluid into the chamber (e.g., infusion of the atmosphere into a low pressure treatment chamber).
Cross-directional flexible seals with leakage control according to the present invention are desirable for the entrances and exits to enclosed pressurized web treatment chambers, such as steam heating chambers, while machine-direction edge seals and a corresponding control system are desirable for other web treatment devices.
Hence, in one aspect, the present invention resides in a control system for detecting and reducing fluid leaks along a seal between a moving web and a web treatment chamber, wherein the web treatment chamber applies a fluid at a pressure other than the ambient pressure to a surface of the moving web, the control system comprising:
a) a leak detector for indicating the presence and location of a fluid leak between the moving web and the seal; and
b) localized leak reduction means responsive to the localized leak detector means.
In another aspect, the invention resides in a method for reducing leaks in a pressurized web treatment system for treating a moving web with a pressurized fluid, the web treatment system comprising a pressurized chamber, and a seal between the web and the chamber, comprising:
a) detecting a leak along the seal, the leak having an estimated severity above a predetermined minimum value,
b) identifying the approximate location of the leak;
c) generating a signal indicative of the approximate location of the leak; and
d) increasing local sealing pressure above a predetermined minimum along the seal in the approximate location of the leak responsive to the signal, such that leakage is reduced.
The above method can further comprise the step of incrementally reducing the local sealing pressure in the region where sealing pressure was increased until leakage in said region begins to increase, then increasing the sealing pressure again slightly to a level before increased leakage was detected, whereby excessive sealing pressures are avoided when not needed to reduce leakage.
In another aspect, the invention resides in a pressurized web treatment system for applying a pressurized fluid from a fluid source to a moving web while dynamically reducing fluid leaks to the atmosphere, comprising:
a) a web treatment chamber comprising chamber walls that define an interior plenum, a fluid inlet for receiving pressurized fluid from a fluid source, an opening for applying pressurized fluid to a moving web, a leading edge and a trailing edge both extending in the cross direction, opposing side edges extending in the machine direction, and a flexible seal assembly along at least one of the leading edge, the trailing edge, and the side edges, for preventing leakage of the pressurized fluid, the seal assembly comprising a first seal head in contact with the web treatment chamber and an opposing second seal head such that the web passes between the first and second seal heads and wherein one of the first and second seal heads is urged toward the other of the first and second seal heads with a sealing force; and
b) a control system for reducing fluid leaks along the seal assembly comprising a leak detector for identifying the presence and location of a fluid leak between the moving web and the seal assembly, and localized leak reduction means responsive to the leak detector, wherein the localized leak reduction means comprises a force generator for variably adjusting the sealing pressure along the length of the seal assembly to reduce leakage in the vicinity of the fluid leak as identified by the leak detector.
In yet another aspect, the invention resides in a treatment system for a moving web with improved leakage control, the web having a first surface and a second surface, comprising:
a) a chamber comprising a plenum for pressurized fluid in fluid communication with the web, the chamber having at least one edge along which the web travels into or out of the chamber;
b) a flexible seal assembly in contact with said at least one edge for preventing leakage of pressurized fluid from the treatment system;
c) a leak detector responsive to fluid leaks along the seal such that the approximate location of fluid leaks can be identified; and
d) localized leak reduction means along the seal assembly cooperatively associated with the leak detector such that increased sealing force is applied where leaks occur.
In yet another aspect, the invention resides in an air press for dewatering a moving web having improved CD control of leakage, comprising:
a) an air press having at a pressurized gas chamber and least one CD seal element;
b) a localized leak detector external to the pressurized gas chamber; and
c) a localized force generator to increase the pressure on the seal, the force generator being cooperatively associated with the leak detector.
Chambers that treat moving webs at pressure other than ambient pressure and particularly at elevated pressure can be generally described, in the context of the present invention, as stationary or non-rotary plenums having a gas inlet to provide a treatment gas or vapor which can contact the web. Contact of the gas with the web is typically by means of the gas flowing through an opening or multiple openings in a treatment face of the chamber that conducts fluid from the plenum to the moving web. The treatment face may be substantially flat or curved. Desirably, it is non-rotary (i.e., it does not rotate continuously during use as does a rotating roll) and substantially static or stationary. Desirably, the web passes through the treatment chamber in a substantially flat form and particularly with a radius of curvature of greater than 30 inches and more particularly with a radius of curvature of greater than about 60 inches.
In many embodiments, the treatment gas in intended to at least partially pass through the thickness of the web, though for steam heating and treatment with reactive chemicals it is not necessary for the gas or vapor to fully pass through the web. When the leakage through open gaps at the entrance, exit, or sides is unacceptable, sealing at any line or region of potential leakage can be provided by placing upper and lower seal elements respectively above and beneath the moving web.
The seal elements are desirably movable or flexible such that they can be brought into contact with the web or with the fabric, wire, or belt on which the web may reside. For example, for treatment of strong movable webs such as a moving textile web, there may be no need for a supporting wire or fabric and the seals may directly contact the moving web. In other cases, the web may require the support a fabric, wire, or belt, such that one seal element contacts the supporting fabric, wire, or belt, while the other opposing seal element contacts the moving web directly. For weaker or more easily damaged materials such as a moist paper web, direct contact with a seal element, particularly a stationary seal element, could result in damage to the web. Thus, it is desirable that certain weak webs such as moist paper webs be sandwiched between two moving fabrics, wires, or belts, to protect the web from direct contact with seal elements. In any case, the combination of the web and any adjacent fabrics, belts, or wires passes through opposing upper and lower seal elements that extend across the leakage zone (e.g., across the cross-direction width of the web for a CD seal element or along the length of the treatment chamber for an MD seal element) and that prevent excess leakage from the pressurized treatment chamber.
For the purposes of the present invention, at least one of the opposing seal elements along a sealing zone is locally deformable, flexible, or locally movable, such that the seal element can respond to an increase in a localized driving force to preferentially adjust the gap between the opposing seal elements in the region of the locally applied force. Similarly, the seal element can respond to the urging or biasing of an actuator or to a change in the position of an actuator or position element attached to or in contact with the seal element to preferentially move the seal element toward and/or away from the web only in the region of the leak. Thus, the sealing means for a treatment chamber comprise a deformable, flexible, or locally movable seal element responsive to locally adjustable force generators or position controllers or means therefor.
Deformable seal elements comprise those of the previously referenced Hada et al. patent application. Seal elements can be a single strip or beam of flexible material such as rubber, Teflon (duPont de Nemours and Co. Inc., Wilmington, Del.) and related fluoropolymers, polyethylene or polypropylene, or the like, or thin metallic beams, rods, or shells capable of suitably flexible deformation under application of force to serve as a sealing element. In one embodiment, the seal element comprises a series of slidable sections joined together but capable or independent or substantial independent deformation or positioning in the sealing direction (i.e., the direction normal to the axis of the seal zone and substantially normal to the moving web in the area being sealed). For example, slidable seal sections can be discrete sections that are interlocked mechanically, for example by a dovetail geometric shape the prevents adjacent sections from separating but permits relative motion in the sealing direction. Slidable elements also can be individually mounted on rods or bearing elements to prevent lateral motion while permitting motion in the sealing direction. Useful support elements for a deformable seal element include interlocked, slidably movable units driven by local actuators or pistons; flexible solid or hollow beams; or the like.
Useful means for applying local sealing force include those systems for adjusting flexible elements known in the art of crepe blade control in tissue making, in the art of slice lip control for headboxes in papermaking, and in the art of press nip control in elongated nips or extended nips known in the art of wet pressing for papermaking. As is known in these and other arts, a wide variety of force generators can be used to drive, bend, flex, move, or reposition a beam, a portion of a beam, or a slidable section in a beam. The beam supporting or holding the seal elements should be deformable or locally movable to permit a force generation means to adjust the shape or position of the beam locally to enable improved local sealing upon application of appropriate force. The deformation or profile of the seal element is determined by the profile of force applied to it or by the action of position adjustment means that determine the position or displacement of structural elements in contact with the seal element.
Locally adjustable force generators or seal element positioners are used to locally control the sealing effect of the seal element to eliminate leakage where needed. A force generator or a seal element positioner such as a position actuator is locally adjustable if there are multiple regions extending across the zone being sealed where the applied force or position of a member can be adjusted to provide a degree of profiling. There are suitably at least two separately adjustable force generators or seal element positioners, but desirably there are at least three and specifically at least five, more specifically at least 8, and more specifically at least 10 separately adjustable force generators or seal element positioning means.
Examples of force generation means suitable for applying variable local force to a seal element include thermal expanding rods, thermal expanding beam elements, thermal hydraulic actuators, mechanical actuators, electric motors, piezoelectric elements, single chamber or multiple chambered air bags, gas cylinders, pneumatic pistons, hydraulic pistons, thermoelectric actuators, mechanical screws, gear assemblies, pulley assemblies, lever and fulcrum assemblies, screw and jack assemblies, adjustable spring assemblies, magnetic force generation systems, or the like. Motors can also be used, such as electric or combustion-driven motors adapted for applying torque or force, particularly when the motor is only required to run momentarily to jog an element into a suitable position, wherein the element can be locked into place by a ratchet and pawl or other means. An example of force generation means coupled to a deformable element is the hydraulically loaded shoe in a typical extended nip or elongated nip press, such as that disclosed by Holopainen in U.S. Pat. No. 5,620,566, issued Apr. 15, 1997, incorporated herein by reference. Holopainen teaches a cross-direction support shoe cooperating with a circular shell serving as a press roll. The support shoe is movably connected to a seat, and between the seat and support shoe are a series of sealed chambers that can receive pressurized fluid. By varying the pressure in each of the chambers, the force applied to the support shoe can be locally varied. The support shoe can be a deformable material to respond more fully to the force profile provided by the variable pressure in the sealed chambers, thus providing a degree of pressure profiling applied to a press nip. More generally, for the present invention, a seat can cooperate with a deformable head via variable pressure or force generation means therebetween to provide a pressure profile to the deformable head, the head being either attached to or cooperating with a seal element. In this case, the portion of the seal element contacting the web or wire may be rigid or deformable, for it is the underlying movable head that is deformable to achieve profiling.
Seal element positioning means can comprise all the previously mentioned force means adapted to move one body relative to another by overcoming a force. Other means for adjusting the position of one body relative to another may be used, including electromotive systems, magnetic levitation, rack and pinion systems, and the use of positioning elements slidably mounted on bearings or rods that can be moved into a desired position.
The deformable or movable seal elements are deformed or moved locally upon application of appropriate force in response to a signal or information provided by local leak detection means. Thus, if local leak detection means indicate that excess gas leakage is occurring in the central portion of a seal, local sealing force in the central portion of the seal would be increased to reduce leakage in that region.
Without wishing to be limited by specific examples, illustrative leak detectors suitable for the present invention include but are not limited to the following:
1. Acoustic sensors such as microphones to detect the presence of leaks based on the sound emitted. An array of microphones or transducers, either sonic, ultrasonic, or subsonic, can be mounted on the plenum, adjacent the plenum, remote from the plenum, or in acoustic communication with the plenum, such that the sensors respond to sound waves created by the dynamics of the leaking stream. In many cases, acoustic sensors covering a frequency of about 1000 Hz to 15,000 Hz will be suitable. Since important information about the size and intensity of the leak can be extracted from the combination of sound intensity and its frequency range or power spectrum, it is desired in some applications that both frequency and intensity information be obtained.
2. Vibration detection systems such as piezoelectric devices or accelerometers responsive to localized motion of solid surfaces on or adjacent to the treatment chamber that can give non-baseline readings in the presence of leaks. Laser-based Doppler anemometers, speckle interferometers, or the like can also be used to assess vibration of a surface.
3. Thermal flow detection means based on the increased heat transfer that occurs in higher velocity flow or based on a temperature difference between the ambient fluid and the pressurized or depressurized fluid in the plenum. In the presence of increased velocity flow, the increased heat transfer coefficient can be sensed with heat flux detectors or hot wire anemometers. Psychrometers, wet bulb thermometers, wetted surfaces, and other devices can indicate the presence of increased velocity gas flow due to the improved evaporative transport or cooling that occurs. When fluid at an elevated or depressed temperature relative to the ambient atmosphere escapes from the plenum, it will result in flows having different temperatures which can be measured by thermometers, thermocouples, thermistors, infrared detectors, liquid crystal thermal displays, other known temperature sensors, thermography with IR-sensitive cameras, or the like. Likewise, if leakage from the atmosphere into a depressurized plenum occurs, the increased heat transfer caused by the in-rushing air or the local temperature changes caused by the atmosphere can be detected with temperature or heat flux measurement means, preferably mounted inside the plenum near the region of potential leakage or mounted on or in the seal elements. Thermal flow detection means can also be responsive to the temperature change that can occur as a pressurized gas passes through a narrow orifice, such as the Joule-Thompson effect.
4. Optical sensors that respond to the gap thickness between opposing seal surfaces to detect changes in light transmission through the edges of the pressurized plenum, particularly in conjunction with suitable light sources. For example, a CCD strip inside a plenum could respond to light from a bright line of lights or a fluorescent tube outside the plenum with suitable light guides and optics to identify the location of the light leak.
5. Chemical sensors, particularly gas sensors for a particular chemical species, that respond to a major or trace component in the treatment gas which may leak into the ambient atmosphere and be detected, or that respond to oxygen or other components of air when entry of the atmosphere into a treatment chamber is to be detected.
6. Gap size measuring devices to directly measure the gap between two opposing seal surfaces or between a seal and a web or fabric, wherein gap size is maintained to a preset value or adjusted in response to leakage detection. Eddy current sensors can also be used to measure the distance between metal components inside two opposing sealing surfaces to assist in regulating and controlling gap thickness at the seals. Gap size can be correlated to leakage or leakage tendency.
7. Mechanical indicators of local flow such as a series of vanes or other rotating devices capable of moving in response to the presence of a local leak, or strips of flexible material such as strings or cloth secured at at least one end and unsecured over a portion of the length of the article such that waving or flapping is possible in response to a leak, to provide a visual cue that leakage is occurring. Flaps, ribbons, and other materials movable in response to air flow can be used.
8. Gas flow or velocity measurement devices such as laser anemometry, hot film or hot wire anemometers, other optical velocimeters, small turbine or vane anemometers, or other such devices to measure the presence of leaks directly in the location of a potential leak zone. In the case of laser Doppler anemometry, the optics could be scanned to move the focal point or measurement volume of the device along a finite zone where leakage may occur. Alternatively, the entire anemometer could be physically scanned, or multiple lasers and optical systems could be used to examine multiple regions.
9. Flow visualization means can be applied to make a local leak visible. For example, a film of surfactant-treated liquid applied near leakage zones could result in a bubble or a visible stream of bubbles near the leak. Likewise, dust, particulates, entrained droplets, aerosols, or other non-gaseous material near the zone of leakage or periodically applied near the zone of potential leakage can provide a visual cue of leakage due to the plumes or visible flow streams created by entrainment or disruption of the materials. The escaping fluid itself may be visible as it escapes from the plenum or may be made visible by means of adding, if only at discrete intervals in time, an amount of a particulate or condensable stream such as steam or an aerosol such as fog or oil droplets or an entrained solid phase such as TiO2 or dust. The visible or visualizable escaping (or, for depressurized system, in-rushing) flow can be detected in any way known in the art, such as by image analysis of CCD camera images. The visible stream can also be sensed optically by the changed intensity of a light beam passing across the potential leakage zone as sensed by a photodetector or optical eye.
For detection of leaks from a pressurized chamber, leak detector sensors or systems will generally be external to the pressurized plenum (though some optical and flow-based detection means may require or be successful with internal probes or sensing devices mounted inside the plenum). For detection of leaks of air or other gases into a depressurized plenum, leak detection sensors can typically be mounted inside the plenum, particularly for oxygen sensors detecting air entry into a chamber.
In a typical embodiment, a moving web is treated in a pressurized chamber. The web may be a web of textile; a woven or nonwoven fabric; a web of papermaking fibers; a polymeric film such as an apertured film; a film undergoing aperturing; an impervious film being treated on one surface at elevated fluid pressure; a metallic foil or sheet; a mat of vegetable matter; a thin wood composite; or the like.
In one application, the moving web comprises activated carbon fibers or a precursor for preparation of activated carbon fabrics. Particularly, the web can be a mat of woven or nonwoven fiberglass or other temperature-resistant inert web impregnated with a phenolic resin preferably further comprising a crosslinking agent such that the resin can be charred while on the inert web to form a carbonaceous coating on the inert web. The charred web can then be treated in a heated gas treatment chamber to activate the carbon and provide desired surface chemistry. Gases such as hydrochloric acid, ammonia, ozone, and other compounds can be used. Acidic groups can be added to the surface of the activated carbon by treating the fibers at elevated temperature in the presence of steam, carbon dioxide, nitric acid, or the like. Basic groups, useful for absorbing acidic compounds such as HCl, can be introduced by treatment with ammonia at elevated temperatures or by other treatments known in the art. Suitable fibers and fiber treatment methods include those disclosed in PCT patent application, xe2x80x9cCoated Absorbent Fibers,xe2x80x9d by James Economy and Michael Daley of the University of Illinois, published as WO 96/38232, Dec. 5, 1996, and on the Univ. of Illinois Web site at xe2x80x9chttp://www.students.uiuc.edu/xcx9cahall/activated carbon fabrics.htmlxe2x80x9d as of Jun. 1, 1998, which discloses a variety of gas treatments at elevated temperature to activate the fibers and control the surface chemistry. The gases should be at a pressure above atmospheric pressure to resist entry of air and should pass completely through the charred web to treat the carbon. Seals are needed for the web as it enters and leaves the heated gas treatment chamber to prevent excess consumption of treatment gas and to prevent oxygen entry. The seals and control system of the present invention can be used for treatment of such webs for the production of activated carbon fabrics. Such activated carbon fabrics are useful for odor control and particularly for incorporation into an absorbent article, for filtration of gases, for use as face masks for medical purposes, for absorption of chemical warfare agents, or the like.
The seal control system of the present invention can also be used in steam boxes for heating and drying of paper webs and textiles, for displacement dewatering of wet webs wherein gas displaces liquid water, for pressurized units in which heated gas is used to create apertures in polymeric films for subsequent use as cover materials in absorbent articles, for particle entrainment in nonwoven and woven webs, for ozonation of textile, paper, and other webs, including disinfectant treatments; or the like.
The control method of the present invention can provide improved means for prevention of leaks in web treatment systems of all kinds wherein a moving web passes through a pressurized treatment chamber. For example, the leakage control system of the present invention is not only suitable for pressurized treatment chambers in which gas passes through a web, but for pressurized treatment chambers where gas at elevated pressure is used to react with or heat a web, particularly for drying. Several examples of a steam treatment chamber for drying a web are found in xe2x80x9cCondebeltxe2x80x9d drying systems, wherein a paper web is heated on one side by a metal band and the web is cooled on the other side, as disclosed in the following patents: U.S. Pat. No. 5,594,997 issued Jan. 21, 1997 to Lehtinen; U.S. Pat. No. 4,899,461 issued Feb. 13, 1990 to Lehtinen; U.S. Pat. No. 5,594,996 issued Jan. 21, 1997 to Haavisto; and U.S. Pat. No. 4,958,444 issued Sep. 25, 1990 to Rautakorpi et al.; all of which are incorporated herein by reference. Cross-directional flexible seals with leakage control according to the present invention are desirable for the entrances and exits to pressurized web treatment chambers, such as the steam-heated chambers of Haavisto, while machine-direction edge seals and a corresponding control system are desirable for the apparatus disclosed by Lehtinen.
The disclosed control system may also be used with single CD seal elements, meaning that the treatment chamber only has one side with a CD seal. The need for a single seal can arise in a variety of cases where a roll of a web such as a textile resides in a pressurized chamber from which the web is unwound and withdrawn without completely relieving the pressure in the chamber. The objective, then, is to maintain a CD seal to reduce pressure loss from the chamber. The need for only one CD seal can arise when a web is formed or created under pressurized conditions and is removed from a reaction chamber without the need for a corresponding seal on the web going into the chamber because the web as such does not go into the chamber but is formed therein.
Alternatively, some pressurized treatments may have two CD seals but only one seal requires improved sealing or CD seal control. Likewise, a treatment may have more than two CD seal sections having CD control.
The principles of the present invention extend not only to CD leak control but also to MD leak control. Specifically, the seal extending along the side edges of a pressurized web treatment device may also be subject to leakage in particular locations and may also benefit from localized leakage detection and control. Certainly CD seals tend to have greater length than MD seals in many web handling situations because there is a trend to make machines increasingly wider. Nevertheless, the finite extent of MD seals in web treatment devices also gives rise to nonuniform leakage spots and makes it advantageous to provide selective local adjustment of sealing force on or the position of seal elements to prevent leakage without excessive wear of seal components, fabrics, or other elements due to unnecessary clamping or excessively high force or friction.
The MD seals can be flexible lips that engage any support fabrics for the moving web along the extreme sides of the fabrics, with air hoses, air bags, solenoids, hydraulic cylinders, pneumatic cylinders, screws, and other known force generating means being used to provide the possibility of profiling in sealing force along the extent of the MD seal. As with CD seals, leakage detection means can identify the location of leaks and send a signal to the force generating means to adjust the sealing pressure in the region of the leakage. The air press system of Hada et al. in U.S. application Ser. No. 08/961,915, filed Oct. 31, 1997 provides examples of MD seals as well as CD seals.
One aspect of the present invention concerns an air press for noncompressively dewatering the wet web. The air press is disclosed in the commonly owned copending application of Hada et al., previously incorporated by reference. The air press is a particularly desirable apparatus for dewatering an uncreped throughdried web to about 30 percent consistency or greater prior to a differential speed transfer. While pressurized fluid jets in combination with a vacuum device have previously been discussed in the patent literature, such devices have not been widely used in tissue manufacturing. A disincentive to using such equipment is believed to be due to the difficulties of actual implementation, including pressurized fluid leaks. The air press of Hada et al. overcomes many prior objections to such a process, and coupled now with the leakage control system of the present invention, offers significant improvements in the ability to dewater a sheet effectively either prior to through drying or as an enhancement of tissue production with a Yankee dryer. In the latter case, improved tissue dried by a Yankee dryer can be made by imparting texture and bulk to the web with sculpted or contoured fabrics such as the sculpted TAD fabrics of Chiu et al. in U.S. Pat. No. 5,429,686, issued Jul. 4, 1995 and incorporated herein by reference, but the texturized tissue web will have less contact with the Yankee unless pressed so hard as to lose the benefits of the texture imparted by the fabric. Less contact with the Yankee reduces drying rates, so additional drying is needed prior to the Yankee for effective sheet molding and structuring. The air press provides a practical apparatus for dewatering a wet web to consistency levels not previously thought possible at industrially useful speeds without thermal dewatering.
In one embodiment, an air press for dewatering a wet web comprises: support fabrics adapted to sandwich the wet web therebetween and transport the wet web through the air press; a first dewatering device comprising a pair of cross-machine direction sealing members including sealing blades; a second dewatering device comprising a cross-machine direction sealing member formed of a deformable material, the first and second dewatering devices moveable relative to one another and adapted to assume an operating position in which the first and second dewatering devices are operatively associated with one another and at least one sealing blade impinges upon the support fabrics and is opposed on the other side of the support fabrics by the sealing member formed of deformable material; and wherein one of the first and second dewatering devices comprises an air plenum operatively connected to a source of pressurized fluid and the other comprises a collection device operatively connected to a vacuum source.
In another embodiment, an air press for dewatering a wet web according to the present invention comprises: support fabrics adapted to sandwich the wet web therebetween and transport the wet web through the air press; an air plenum positioned on one side of the wet web and operatively connected to a source of pressurized fluid, the air plenum comprising a profitable or segmented sealing assembly that is adapted to move at each segment or section between an operating position and a retracted position, the sealing assembly comprising a pair of machine direction sealing members and a pair of cross-machine direction sealing members that form an integral seal with the wet web when the sealing assembly is in the operating position; a collection device positioned on the opposite side of the wet web and operatively associated with the air plenum, the collection device defining therein sealing slots that extend across the width of the wet web and also defining therein a central passageway disposed between the sealing slots and adapted to receive pressurized fluid from the air plenum and water from the wet web, and means for moving the machine direction sealing members into and out of contact with one of the support fabrics, the machine direction sealing members forming a seal when the sealing assembly is in the operating position.
The air press is able to dewater the wet web to very high consistencies due in large part to the high pressure differential established across the web and the resulting air flow through the web. In particular embodiments, for example, the air press can increase the consistency of the wet web by about 3 percent or greater, particularly about 5 percent or greater, such as from about 5 to about 20 percent, more particularly about 7 percent or greater, and more particularly still about 7 percent or greater, such as from about 7 to 20 percent. Thus, the consistency of the wet web upon exiting the air press may be about 25 percent or greater, about 26 percent or greater, about 27 percent or greater, about 28 percent or greater, about 29 percent or greater, and is desirably about 30 percent or greater, particularly about 31 percent or greater, more particularly about 32 percent or greater, such as from about 32 to about 42 percent, more particularly about 33 percent or greater, even more particularly about 34 percent or greater, such as from about 34 to about 42 percent, and still more particularly about 35 percent or greater.
The air press is able to achieve these consistency levels while the machine is operating at industrially useful speeds. As used herein, xe2x80x9chigh-speed operationxe2x80x9d or xe2x80x9cindustrially useful speedxe2x80x9d for a tissue machine refers to a machine speed at least as great as any one of the following values or ranges, in feet per minute: 1,000; 1,500; 2,000; 2,500; 3,000; 3,500; 4,000; 4,500; 5,000, 5,500; 6,000; 6,500; 7,000; 8,000; 9,000; 10,000, and a range having an upper and a lower limit of any of the above listed values. Optional steam showers or the like may be employed before the air press to increase the post air press consistency and/or to modify the cross-machine direction moisture profile of the web. Furthermore, higher consistencies may be achieved when machine speeds are relatively low and the dwell time in the air press in higher.
The pressure differential across the wet web provided by the air press may be about 25 inches of mercury or greater, such as from about 25 to about 120 inches of mercury, particularly about 35 inches of mercury or greater, such as from about 35 to about 60 inches of mercury, and more particularly from about 40 to about 50 inches of mercury. This may be achieved in part by an air plenum of the air press maintaining a fluid pressure on one side of the wet web of greater than 0 to about 60 pounds per square inch gauge (psig), particularly greater than 0 to about 30 psig, more particularly about 5 psig or greater, such as about 5 to about 30 psig, and more particularly still from about 5 to about 20 psig. The collection device of the air press desirably functions as a vacuum box operating at 0 to about 29 inches of mercury vacuum, particularly 0 to about 25 inches of mercury vacuum, particularly greater than 0 to about 25 inches of mercury vacuum, and more particularly from about 10 to about 20 inches of mercury vacuum, such as about 15 inches of mercury vacuum. Both pressure levels within both the air plenum and the collection device are desirably monitored and controlled to predetermined levels.
The collection device desirably but not necessarily forms an integral seal with the air plenum and draws a vacuum to facilitate its function as a collection device for air and liquid. The terms xe2x80x9cintegral sealxe2x80x9d and xe2x80x9cintegrally sealedxe2x80x9d are used herein to refer to: the relationship between the air plenum and the wet web where the air plenum is operatively associated and in indirect contact with the web such that about 70 percent or greater of the air fed to the air plenum flows through the web when the air plenum is operated at a pressure differential across the web of about 30 inches of mercury or greater; and the relationship between the air plenum and the collection device where the air plenum is operatively associated and in indirect contact with the web and the collection device such that about 70 percent or greater of the air fed to the air plenum flows through the web into the collection device when the air plenum and collection device are operated at a pressure differential across the web of about 30 inches of mercury or greater.
Significantly, the pressurized fluid used in the air press is sealed from ambient air to create a substantial air flow through the web, which results in the tremendous dewatering capability of the air press. The flow of pressurized fluid through the air press is suitably from about 5 to about 500 standard cubic feet per minute (SCFM) per square inch of open area, particularly about 10 SCFM per square inch of open area or greater, such as from about 10 to about 200 SCFM per square inch of open area, and more particularly about 40 SCFM per square inch of open area or greater, such as from about 40 to about 120 SCFM per square inch of open area. Desirably, 70 percent or greater, particularly 80 percent or greater, and more particularly 90 percent or greater, of the pressurized fluid supplied to the air plenum is drawn through the wet web into the vacuum box. For purposes of the present invention, the term xe2x80x9cstandard cubic feet per minutexe2x80x9d means cubic feet per minute measured at 14.7 pounds per square inch absolute and 60 degrees Fahrenheit (xc2x0 F.).
The terms xe2x80x9cairxe2x80x9d and xe2x80x9cpressurized fluidxe2x80x9d are used interchangeably herein to refer to any gaseous substance used in the air press to dewater the web. The gaseous substance suitably comprises air, steam or the like. Desirably, the pressurized fluid comprises air at ambient temperature, or air heated only by the process of pressurization to a temperature of about 300xc2x0 F. or less, more particularly about 150xc2x0 F. or less.
In an alternative embodiment, a device for dewatering a wet web traveling in a machine direction, comprises: a frame structure; support fabrics adapted to sandwich the wet web therebetween; an air press comprising an air plenum and a collection device positioned on opposite sides of the wet web and support fabrics, the air plenum and collection device operatively associated with one another and adapted to establish a flow of pressurized fluid through the wet web, the air plenum comprising: stationary components mounted on the frame structure; a profitable sealing assembly that is adapted to move relative to the stationary components between an operating position and a retracted position with variation in position of segments or sections along the sealing assembly providing a degree of independent control over each segment or section. The sealing assembly comprises a pair of optionally profitable machine direction sealing members and a pair of profitable cross-machine direction sealing members that together form an integral seal with the wet web when the sealing assembly is in the operating position; means for moving the cross-machine direction sealing members generally perpendicular to a plane containing the wet web and into and out of contact with one of the support fabrics; means for moving the machine direction sealing members generally perpendicular to the plane containing the wet web and into and out of contact with one of the support fabrics; and means for moving the machine direction sealing members generally parallel to the plane containing the wet web and generally perpendicular to the machine direction.
In another alternative embodiment, a device for dewatering a wet web traveling in a machine direction, comprises: a frame structure; support fabrics adapted to sandwich the wet web therebetween; an air press comprising an air plenum and a collection device positioned on opposite sides of the wet web and support fabrics, the air plenum and collection device operatively associated with one another and adapted to establish a flow of pressurized fluid through the wet web, the air plenum comprising: stationary components mounted on the frame structure and defining a loading surface generally parallel to a plane containing the wet web; a sealing assembly that is adapted to move relative to the stationary components between an operating position in which the sealing assembly forms an integral seal with the wet web and a retracted position, the sealing assembly defining a control surface generally parallel to the plane containing the wet web and adapted to contact the loading surface; and means for moving the sealing assembly generally perpendicular to the plane containing the wet web, wherein contact between the control surface and the loading surface interrupts movement of the sealing assembly toward the wet web when the sealing assembly reaches the operating position.
The air press may use internal surfaces that are normal to the loading direction to completely isolate the loading force from the air plenum pressure. Thus, the loading force can be maintained at a constant value to provide a proper seal despite the air plenum pressure varying from zero to maximum pressure. Accordingly, the loading force does not have to be adjusted in response to pressure changes within the air press.
As used herein, xe2x80x9cfluidxe2x80x9d refers to non-solid matter, specifically liquids or gases. Though gas is most commonly used in pressurized web treatment chambers, liquids can also be applied. Liquids that could be used according to the present invention include water, resins to make impregnated paper, dyes, surface chemistry modifying solutions, supercritical fluids (classifiable as either a liquid or a gas), solvents for removing undesired components, or the like.
As used herein, xe2x80x9cgasxe2x80x9d refers to a chemical composition which is not in a liquid or solid state, but is gaseous. Exemplary gases of use in the present invention include air, steam, nitrogen, oxygen, ozone, chlorine, bromine, and other halogens, acid vapors such as HCI or sulfuric acid fumes, ammonia and nitrogenated compounds, chlorinated species such as bleaching agents or dry cleaning compounds, solvent vapors, formaldeyde and other crosslinking agents, alcohols, or the like, either as pure components or in mixtures.
As used herein, xe2x80x9cvaporxe2x80x9d refers to a readily condensable gas or the gases resulting from evaporation or boiling of a substance, with examples including steam, aliphatic fumes, many acid fumes, fumes of many heavy metal organic compounds in the gaseous state, particularly titanium and osmium compounds, sublimed material, or the like. The gases or vapors used to treat webs may also carry entrained or suspended liquid or solid materials such as superabsorbent particles; pH modifying agents or buffer compounds, activated carbon particles or fibers; synthetic or natural polymeric fibers or particulates; oils and emollients; dust; fume particles; baking powder; skin wellness agents such as zinc oxide or dimethicone; surfactants; bleaching agents; colors, dyes, and pigments; inorganic fillers such as calcium carbonate, titanium dioxide, chalk, talcum; or the like.
As used herein, xe2x80x9ccross directionxe2x80x9d (CD) refers to the direction substantially normal to the machine direction (the direction of movement) of a moving web. A seal element is said to run substantially in the cross-direction of the web if it extends across a greater distance normal to the machine direction than it extends in the machine direction. Desirably, the CD seal extends substantially across all of the width of a web.
As used herein, xe2x80x9cLocal Sensitivityxe2x80x9d is the percentage of the linear extent of a sealed region that is occupied on the average by a separately controllable force generator or position controlling device. For example, a spaced apart linear array of 5 independently adjustable pistons or pressurized hose segments along a cross-direction seal could apply CD force profiling across the sealing zone with a Local Sensitivity of 20%.